1. Field of the Invention
The present invention relates to a method of fabricating a double-sided printed circuit board, and more particularly, to a method of fabricating a double-sided circuit board with superfine circuits and high density circuits.
2. Description of the Prior Art
In recent history, printed circuit boards (PCBs) have been widely used for carrying various electronic components and devices. As there is demand for electronic products to be lighter, smaller, and portable, research and development of printed circuit boards is unavoidably moving toward a direction of miniaturization, higher integration, lower thickness, and the use of multiple layers.
In general, printed circuit boards having circuit layouts on both sides of the circuit boards have been widely used in numerous electrical apparatuses such as air-conditioners, telephones, and fax machines. Nevertheless, it is essential to have a “bridge”, or otherwise referred to as through holes, for communicating between the circuits and electrical wires on both sides of the board. The useable area of a typical double-sided printed circuit board is often twice that of a typical single-sided printed circuit board, and in contrast to a single-sided printed circuit board, the double-sided printed circuit board is more suitable to be used for products with complex circuits.
Please refer to FIG. 1. FIG. 1 is a perspective diagram showing a double-sided printed circuit board after a mechanical drilling process is performed according to the prior art. In the past, the mechanical drilling processes have always been performed on a substrate 30 covered with a copper clad 32 (together referred to as a copper clad laminate, CCL). The limit of the conventional mechanical drilling technique is about 75 micrometers (diameter of the drilled hole on CCL). It is very difficult to form a drilled hole with diameter smaller than 75 micrometers by conventional mechanical drilling technique. In order to drill through the copper clad 32 and the substrate 30, enormous time and effort have to be spent. Consequently, the mechanical drilling will increase the diameters of the through holes 34, thereby reducing the amount of useful space. Moreover, the thickness of the substrate 30 will also increase significantly if additional patterns need to be added. Hence it becomes a major disadvantage for the traditional method to fabricate printed circuit boards with superfine wires and circuits with higher integration. There is a need in this industry to provide a method of forming a through hole with smaller dimensions on the PCB substrate.